Environmental Health - Toxic Substances
Biofilms in Streams Help Create Daily Variations in Metal Concentrations
This greenish-brown slime found on cobbles of the streambed in High Ore Creek, Mont., is a biofilm. USGS scientists and their colleagues have demonstrated that biofilms can create daily variations in the concentrations of dissolved metals in stream waters
U.S. Geological Survey (USGS) scientists and their colleagues have demonstrated that the large daily changes in zinc concentrations observed in mining-affected streams are in some cases caused by biofilms. Biofilms are layers of greenish-brown slime commonly found on rocks and plants in streams. They are composed of microscopic bacteria, fungi, diatoms, and other algae. The scientists conducted a series of laboratory and field experiments that documented biofilms absorbing zinc from stream water during daylight hours and then releasing zinc to the stream during the night. This alternating absorption and release can create the daily changes in zinc concentrations observed in streams. Scientists, regulators, land managers, and reclamation teams working in metals-contaminated streams need to be aware of this daily metal cycling to ensure that monitoring and assessments of aquatic toxicity are accurate. The highest zinc concentrations (and therefore, perhaps the most toxic conditions) in stream water might occur only at night, and the greatest exposure to aquatic invertebrates (insects and worms), which feed on biofilms, might only occur during daylight hours.
Results of a 54.5-hour field experiment on the banks of High Ore Creek, Mont., conducted in August 2002. Biofilm material grown in a laboratory without exposure to zinc was added to a streamside aquarium. Filtered stream water was then pumped through the aquarium, and the aquarium was exposed to natural sunlight. The graph shows zinc concentrations measured in water (red triangles) and the biofilm (blue circles) in the aquarium. The graph is a modified version of figure 3 from Morris and others, 2005
Farag, A.M., Nimick, D.A., Kimball, B.A., Church, S.E., Harper, D.D., and Brumbaugh, W.G., 2007, Concentrations of metals in water, sediment, biofilm, benthic macroinvertebrates, and fish in the Boulder River Watershed, Montana, and the role of colloids in metal uptake: Archives of Environmental Contamination and Toxicology, v. 52, no. 3, p. 397-409, doi:10.1007/s00244-005-0021-z.
Morris, J.M., Farag, A.M., Nimick, D.A., and Meyer, J.S., 2006, Light-mediated Zn uptake in photosynthetic biofilm: Hydrobiologia, v. 571, p. 361-371, doi:10.1007/s10750-006-0261-6.
Morris, J.M., and Meyer, J.S., 2006, Extracellular and intracellular uptake of zinc in a photosynthetic biofilm matrix: Bulletin of Environmental Contamination and Toxicology, v. 77, no. 1, p. 30-35, doi:10.1007/s00128-006-1028-5.
Morris, J.M., and Meyer, J.S., 2007, Photosynthetically mediated Zn removal from the water column in High Ore Creek, Montana: Water, Air, and Soil Pollution, v. 179, no. 1-4, p. 391-395, doi:10.1007/s11270-006-9232-9.
Morris, J.M., Nimick, D.A., Farag, A.M., and Meyer, J.S., 2005, Does biofilm contribute to diel cycling of Zn in High Ore Creek, Montana?: Biogeochemistry, v. 76, no. 2, p. 233-259, doi:10.1007/s10533-005-4774-2.
Nimick, D.A., 2003, Diurnal variation in trace-metal concentrations in streams: U.S. Geological Survey Fact Sheet 086-03, 4 p.
Glass plates coated with laboratory-cultured biofilm material in an aquarium on the banks of High Ore Creek, Mont. Water from the creek was continuously pumped through the aquarium during an experiment to measure the absorption and release of zinc by the biofilms.
Graph showing daily variation of dissolved zinc concentrations in High Ore Creek, Mont., for five different years. Scientists suspect that biofilms on rocks and plants in streams contribute to the observed changes in concentrations.
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